The present invention relates to a process for electrochemically modifying aluminum or aluminum alloy-based support materials for printing plates, which previously have been electrochemically roughened, and to the use of the materials thus modified in the manufacture of offset printing plates.
Support materials for offset printing plates are provided, on one or both sides, with a radiation-sensitive (light-sensitive) coating (copying or reproduction coating), either directly by the user or by the manufacturer of precoated printing plates, this coating permitting the production of a printing image (printing form) by a photomechanical process. Following the production of the printing form, the coating support carries the printing image areas and, simultaneously, forms, in the areas which are free from an image (non-image areas), the hydrophilic image background for the lithographic printing operation.
The following requirements must, among others, be met by a support for a radiation-sensitive material of this type used in the manufacture of lithographic plates:
Those portions of the radiation-sensitive coating which are comparatively more soluble following irradiation must be capable of being easily removed from the support, by a developing operation, in order to produce the hydrophilic non-image areas without leaving a residue.
The support, which has been laid bare in the non-image areas, must possess a high affinity for water, i.e., it must be strongly hydrophilic, in order to accept water, rapidly and permanently, during the lithographic printing operation, and to exert an adequate repelling effect with respect to the greasy printing ink.
The photosensitive coating must exhibit an adequate degree of adhesion prior to exposure, and those portions of the coating which print must exhibit adequate adhesion following exposure.
The support material should possess a good mechanical stability, for example, against abrasion and a good chemical resistance, particularly with respect to alkaline media.
Water requirement during printing should be as low as possible, for example, to prevent excessive moistening of the paper because, otherwise, "register difficulties" in color work (i.e. the second or third color shade can no longer be printed in register upon the first color shade) or breaks in the paper web in rotary offset printing may occur.
In order to meet some of these requirements, support materials of aluminum, which are conventionally employed in practice, are, first of all, subjected to a mechanical, chemical and/or electrochemical roughening treatment, which additionally may be followed by an anodic oxidation of the roughened aluminum surface. Particularly, electrochemically roughened aluminum surfaces with their very fine-grained structure forming an interface between the support material and the radiation-sensitive coating of printing plates, produce, in the printing forms which can be manufactured from these plates, results which meet practical requirements and which already comply with most of the demands. Water requirements during printing are, however, often still too high in the support materials which have been roughened and optionally anodically oxidized according to known processes. Modifications of these processes have, therefore, already been described which especially may be applied after the roughening step and which, for example, include the following processes:
German Offenlegungsschrift No. 3,009,103 (equivalent to South African Pat. No. 81/1545) discloses an abrasive modification of electrochemically roughened support materials for printing plates comprising aluminum. In this modifying treatment, an abrasive removal of material from the surface in the order of from 0.4 to 3.0 g/m.sup.2 is effected under the action of an aqueous-alkaline solution which has a pH value exceeding 11. Printing plates manufactured from support materials which have been thus modified and optionally anodically oxidized, are stated to have a lower consumption of dampening solution and a reduced adsorptivity.
A similar process, especially for manufacturing aluminum support materials for positive-working reproduction coatings in the field of printing plates is described in German Offenlegungsschrift No. 3,036,174 (equivalent to British Pat. No. 2,060,923). In a preferred process variant, the support material is mechanically roughened prior to the electrochemical roughening treatment and the surface which has been roughened in this manner is also abrasively modified using an aqueous acid or base. It is stated that a printing plate produced from this support material shows a long press life, a high resistance to staining in the non-image areas and a uniform roughening structure.
In the method of producing printing plate support materials of aluminum according to German Offenlegungsschrift No. 2,557,222 (similar in content to U.S. Pat. No. 3,935,080), the support material is additionally cathodically modified (cleaned) in an aqueous sulfuric acid, between the step of an electrochemical roughening in an aqueous hydrochloric acid and the step of an anodic oxidation in an aqueous sulfuric acid. It is stated that the method is, in the first place, suitable for use in a continuous process and that it results in a very clean surface.
From the prior art, a cathodic treatment is also known for use in other methods:
According to German Auslegeschrift No. 2,420,704 (equivalent to U.S. Pat. No. 3,865,700) cathodic contacting of aluminum supports is used in the anodic oxidation of these supports in an aqueous sulfuric acid, in order to prevent the use of contact rolls, which are normally present.
German Pat. No. 2,537,724 (equivalent to British Pat. No. 1,532,303) discloses a one-step roughening process without subsequent abrasive modification of the surface, in which aluminum support materials for printing plates are electrochemically treated in agitated aqueous salt solutions having a salt concentration of at least 200 g/l, a pH value ranging from 5 to 8 and a temperature of less than 60.degree. C. The salts used are alkali metal salts, alkaline earth metal salts or ammonium salts of hydrohalogenic acids or oxo-acids of nitrogen or of halogens. In a process variant (resulting in surfaces of type A), the aluminum can be roughened, in a cathodic circuit arrangement, for a duration from 30 to 60 seconds with direct current of 70 to 150 A/dm.sup.2, whereby a silvery surface with a dull finish is produced; in this variant, alkali metal salts are exclusively used. Similarly, German Pat. No. 2,537,725 (equivalent to British Pat. No. 1,532,304) describes a possible cathodic circuit arrangement for the roughening of aluminum, in which the aqueous electrolyte, at a pH value ranging from 1 to 5, must contain an alkali metal salt in addition to aluminum salts.
It is true that a chemical or electrochemical modification of electrochemically roughened support materials for printing plates in aqueous acids produces a good cleaning effect on the surfaces thus treated. A marked reduction in the consumption of dampening solution of the printing forms manufactured from such supports is, however, not noticed and, in addition, an adverse influence on the printing run occasionally may be observed with this type of treatment. A chemical modification of electrochemically roughened support materials for printing plates in aqueous-alkaline solutions may often fulfill the above-outlined practical requirements demanded of a printing plate, but it has, nevertheless, some technological disadvantages. Due to the abrasive removal of aluminum and the generation of Al(OH).sub.4.sup.- or AlO(OH).sub.2.sup.- ions connected therewith, the amount of OH.sup.- ions in the solution is constantly reduced, which causes a change in the concentration of OH.sup.- ions in the aqueous-alkaline solution and, as a result of the known inhibiting effect of the aluminate produced, leads to losses of action of the bath and, consequently, to short useful lives of the modifying solution. Monitoring the process to obtain unchanging product characteristics is, naturally, rendered difficult by such concentration variations and also by temperature variations which occur. In addition, the disposal of the relatively aggressive aqueous-alkaline modifying solutions after use thereof is not without problems from the point of view of preventing water pollution.